Laboratory Medicine—Pregnancy Testing



Laboratory Medicine—Pregnancy Testing

Indication for Testing

HCG (human chorionic gonadotropin) urine/blood tests are used to diagnose pregnancy. The newly formed placenta is responsible for the new HCG. Whenever there is living chorionic placental tissue, HCG is present in the blood or urine. Initially, HCG is the blood is higher than the urine. Over time, they equalize. It is also helpful in the monitoring of “high risk” pregnancies. It can also be used as a tumor marker for some cancers in men and in women who are not pregnant (germ cell tumors). HCG levels will become negative 3-4 days after birth or miscarriage.

HCG is similar to pituitary proteins. It is composed of an alpha and beta subunit. The alpha portion of HCG is the same for all glycoproteins. The beta portion is very specific for beta-HCG, so there are no cross reactions. Whole HCG, which contains alpha and beta units, can cross-react with LH. Cross reactions can lead to false positives. All pregnancy tests are based on the detection of HCG in the blood or urine.

Types of Pregnancy Testing

Biologic

1) Inhumane – uses mice, frogs, and rabbits. Urine would be taken from a suspected pregnant woman and injected into the animal. The animal would then be killed and examined for ovulation. If ovulation occurred, the woman would be considered positive for pregnancy

2) Subjective component to reading

3) Not sensitive

4) Not cost effective

5) Not performed anymore

6) Usually positive by the 40th day after the last menstrual period

Immunologic

1) Cost effective – can be purchased OTC. Commercially available

2) Sensitive – most are positive 4-7 days after the first missed period

3) Cross reactivity with LH. Results may be altered by proteinuria, immunologic disease, menopause, and hypothyroidism

4) Results available in 2 minutes-2 hour

5) High false positive rate related to whole HCG. Beta subunit has greatly improved accuracy

Radioimmunologic (Radioimmunoassay) (RIA)

1) Sensitive and specific – detects the beta subunit

2) Requires 24-48 hours incubation time. Newer tests require 1-5 hours

3) Used to determine viability and normalcy of early pregnancy

4) Can diagnose pregnancy BEFORE the first missed period

5) Does NOT cross react with LH

Radioreceptor Assay (RRA)

1) Highly sensitive and accurate – 90-95% accurate 6-8 days after conception

2) Can be performed in 1 hour

3) Cross reacts with LH

The last two tests are used in very sensitive and specific populations (i.e. rape victims). Both require a type of fluorescent tagging. The tag and the maternal serum will compete with sites on the plate. If the woman has positive HCG, the plate will not appear fluorescent, which indicates a positive pregnancy.

Interfering Factors

1) Test may be performed too early to diagnose pregnancy – false negative results

2) Hematuria or proteinuria may cause false positive results

3) Hemolysis of blood may interfere with serum studies

4) Dilute urine may cause levels of HCG to be undetectable – false negative results

5) Drugs may cause false negative results (diuretics and Promethazine). Phenergan is used for motion sickness, nausea, and vomiting

6) Drugs may cause false positive results – anti-convulsants, anti-parkinsonism meds, hypnotics, and tranquilizers.

Conditions Associated with Increased HCG Levels

1) Pregnancy

2) Ectopic pregnancy

3) Hydatidaform mole

4) Choriocarcinoma of the uterus

5) Germ cell tumors of the testes or ovaries – choriocarcinoma, teratomas, embryonal cell

6) Poorly differentiated tumors of the liver– hepatomas and lymphomas

Conditions Associated with Decreased HCG Levels

1) Threatened abortion

2) Incomplete abortion

3) Dead fetus

Whole HCG Measurement Chart

|Gestation (wk) |Whole HCG (mlU/ml) |

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